A plurality of machines for processing substrate webs, for example, paper webs or fabric webs is known. These processing machines can perform a multitude of process steps on the substrate web such as, for example, printing, embossing or cutting. Most process steps require that the substrate web display a tension that is as uniform as possible. If the substrate web is not tensioned uniformly, process errors may occur in the substrate web such as, for example, shifts of printed images, as well as tears or creases. In order to maintain a uniform tension of the substrate web such processing machines comprise tensile stress control arrangements.
In particular in multi-color printing machines, it is necessary that the substrate web tension of a substrate web be controlled as accurately as possible in order to enable printing in a register-perfect manner. The description hereinafter thus refers to the example of a printing machine in greater detail; however, it should be noted that both the device, as well as the method for controlling the substrate web tension, can be used with other processing machines.
A known design of a tensile stress control device comprises a force-measuring box that measures the tension of the substrate web and emits a corresponding measuring signal. The measuring signal is used for the control of an associate driving unit.
Depending on the size of the driving unit that is used for controlling the tensile stress, there are limits regarding the disturbances of the substrate web tension that can be corrected by the driving system. However, the remaining errors and fluctuations are not acceptable, in particular in the case of printing machines because said errors and fluctuations can seriously compromise the printed image.
For example, in reel fed printing machines there are existing strict requirements regarding the achievable tensile stress and the allowed tensile stress errors. It is impossible to meet these requirements with the aforementioned conventional method, wherein a tensile stress control is based on a not specifically processed measured value from force-measuring boxes.
With regard to the dimensioning of drive and control of a printing machine, in most cases the problem occurs that an electric motor with strong torque is necessary for the required mean substrate web tension. However, the rotors of these motors display large moments of inertia and long coil delay times that make it difficult to correct high-dynamic disturbances of the substrate web tension. Here the coil delay time is understood as the ratio of coil inductance to coil resistance. Conversely, it can be said that small electric motors displaying a small moment of inertia of the rotor and low coil delay times provide the dynamics necessary for correcting the highly dynamic disturbances but cannot provide the required torque.